As a typical main circuit construction for inverters, indirect ac power conversion circuits for converting a commercial alternating current into a direct current via a rectifier circuit and a smoothing circuit to obtain an ac output by a voltage-type converter are generally used. Meanwhile, as a method for obtaining an ac output directly from an ac voltage, direct ac power converter apparatuses typified by matrix converters are known. In this case, since large capacitors and reactors for smoothing voltage pulsations by commercial frequencies are unnecessary, converter miniaturization can be expected and therefore the direct ac power converter apparatuses have been increasingly gaining attentions as next-generation power converter apparatuses.
A three-phase to three-phase matrix converter is to obtain ac output voltages of variable voltages and variable frequencies by direct switching of a three-phase ac input voltage. As a typical modulation method, an analog base modulation method in which, with a virtual DC link formed, sinusoidal modulation is performed by carrier comparison is proposed in “VVVF On-Line Control of PWM Cycloconverter”, Jun OYAMA and other five, IEEJ Transactions D, IEEJ, Vol. 116 (1996), No. 6, pp. 664-651. Also, a control method for solving current distortion, which is an issue in asynchronism between output voltage command signal and carrier signal, is disclosed in Japanese Patent Publication No. 11-341807 A.
The control methods of “VVVF On-Line Control of PWM Cycloconverter” and Japanese Patent Publication No. 11-341807 A, in either case, is based on an amplitude modulation which is accomplished by introducing a variable that is a current distribution ratio, i.e. a ratio of a mid phase current to a maximum phase current, and by multiplying a factor on each of an output voltage command signal and a carrier signal. Further, the control method of Japanese Patent Publication No. 11-341807 A involves two steps of control for suppressing changeover of switching patterns, which serve as information updating part, so that current distortion in carrier asynchronism is avoided. Due to this, matrix converters of “VVVF On-Line Control of PWM Cycloconverter” and Japanese Patent Publication No. 11-341807 A have an issue that control configuration and computation become more complex.